The Curie-Becquerel story


Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309. Everyone has heard of Madame Curie. She and he...
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The Curie-Becquerel Story Harold F. Walton Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309

Everyone has heard of Madame Curie. She and her husband Pierre are the subiects of several bio~raohirs.The story of the young woman from Poland whowekt to Paris to studv a t the Sorbonne, who met a Dhvsicist with whom she shared her life and her science &d jived in voluntary poverty, and went on to win two Nobel prizes, is dramatic indeed. What is not so well-known is that the scientific story, the story of the discovery of radioactivity and radium, is just as dramatic. The three principal actors in this story are Henri Becquerel, Marie Sklodowska-Curie, and Pierre Curie. One could put masks over the heads of these three people, like the characters in Japanese Noh drama, and not know that one of them was a woman, and the story would still be great drama, a n exciting case history in scientific discovery.

RBntgen's Contribution I n November 1895, i n Wurzburg, Germany, Wilhelm Conrad Rtintgen discovered X-rays and announced his discovery in December (I).This discovery excited Becquerel. On January 20, 1896, the Academy of Sciences i n Paris saw a photograph ofthe bones of a human band, taken by X-rays. The photograph created great excitement. More photographs were forthcoming, and the question arose, "Could one make the photography faster?" Another question was, "What is the connection between X-rays and nhosnhorescence?" Consider how X-ravs were ~roduced:a stream of cathode rays in a n evacuagd tube bet the glass wall of the tube and made it glow with a bright green phosphorescence. Invisible, penetrating X-rays emanated from this brieht . it be that invisible oeuetratine ravs - s .~ o tCould " always accompanied visible phosphorescence? &

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The Study of the Histow of Science

Science is a n interesting human endeavor, spurred by the curiosity to know what is around the next corner or over the next hill, the urge to know how nature works. Because it is a human endeavor. no two scientists eo about it in quite the same way. The search has its happy accidents where "fortune favors the DreDared mind". I t also has its mistakes and misunderstandings and unexplained loose ends. The Curie-Becquerel story makes a good case history, for it happened sufficiently long ago that we know its ending. At the same time it is sufficiently recent that we can put ourselves in the actors' places. There is another great merit to this case history. Most of the work was published in a single journal, Comptes Rendus de I'Academde des Scdenees. the Transactions of the French Academv of Sciences. A& of us who has access to a good library and a working knowledw of French can take down the volumes of these Transactions from the shelves and read the papers, one after another, and see how the writers thought.

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The 19th Century French Academy of Sciences

I n the late 19th century, when our story begins, the French Academv of Sciences was a cozv. select eentlemen's club in Paris wiih about 60 resident mGmbers Gho met everv Mondav evenine to tell each other what thev had been d&ng. het texts of ;he talks, limited i n length, h e r e given to the secretarv bv the followine Wednesday, then thev were printed aAd iistributed to the membersin time f i r next Mondav's session. Thus, we can see how the scientists were think&. We can see the false starts, the loose ends, and the progression of events. A distinguished member of the Academy was AntoineHenri Becquerel. His father, Alexandre-Edmond Becquerel, was a physicist who had studied crystals and their luminescence and had built a rotating-shutter device for measuring ~hosnhorescentlifetimes. He had studied the luminescenee of'uranium salts in 1872. His grandfather, Antoine Becauerel. was a Dhvsicist too. He had observed piezoel~ctrickyin 1819. A; f i r Henri, his reputation was made. He was professor of the prestigious Ecole Polytechnique and also of the Museum of Natural History. He had no reason to exert himself further unless he so desired. 10

Journal of Chemical Education

Becquerel's Contribution Now it was time for Becquerel to get into the act. He knew that uranium salts were phosphorescent. In ultraviolet lieht thev elow with a beautiful orimrose vellow color. Hc &d t h e i e ~ a l t s o nhis shelf: His iather's work showed the lifetime of nhosohorescence of uranium salts to be less than 11100 s. ( ~ o d awewould ~ call the shortlived glow fluorescence.)In contrast. zinc sulfide has a long ~hosohorence. A somewhat free tianslation of ~ e c ~ u e r i itest ' s results follows. Bec uerel's Re ort to the Academy on #onday, Fe ruary 24, 1896

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At a recent session Mr. Charles Henry announced that phosphorescent zinc sulfide, placed in the path of X-radiation from a Craakes tube. increased the intensitv of radiations that penetrated aluminuk. Mr. ~iewenglowskihoted that calcium sulfide of commerce emits radiations that penetrate opaque bodies. This property extends to other phosphorescent materials, is verv in narticular uranium salts. whose nhosnhorescence . . shdn-lwed. With thcd~rublesulfntr,,furan~um and potassium. nf whwh I have pome crptnls in the form of a thin crust on glass, I made these observations. .One wraps a photographic plate in two thiclmesses of heavy black paper, such that a day's exposure to sunlight produces no blackening of the plate. On the outside of the paper wrapping one places the phosphorescent material, then exposes the package to the sun's rays far several hours. 'On developing the photographic plate one sees the image of the phosphorescent crystals appearing on the platein black. 'If one nuts a coin between the nhosohorescent material . and ch'e paper wrapping or a l n e ~ srnp l n t h a design cur out of i t , one sees the imogrs of these ohjwrr on the photographic plate.

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One can EDeat these ex~erimentswith a thin sheet of elass between the dhosphareseerk material and the paper. 'Thisfact excludes the oossibilitv that the warmth of the snn'a rnvr might have causrd chemically n c t w wpors to come out of t h ~ phusphorrsernt ralt 2nd reduce the silver hramlde of the phatographic plate ~~

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A Change of Scenes With the monograph of April 12, we may say that the curtain falls on Act I of our drama. When it rises on Act I1 the scene has changed. Becquerel had made his experiments in a private laboratory in a beautiful sandstone house in the Botanical Gardens of Paris (Jardin des Plantes), the house in which he had been born, reserved for the Professor of the Natural Hiiitow Museum (31. Now we move to a large red brick school building a kilometer to the west. in an old Dart of Paris. across the narrow street from the school wcere paste& had taught. Our scene is the municipal school of industrial physics and chemistry,then called "Ecole Municipale de Physique et de Chimie Industrielles". This school, comparable to a n American graduate school or a research institute, is today one of the most distinguished and productive research institutions in France. It is now called "Ecole Superieure de Physique et de Chimie de Paris". As one enters the courtyard from the street, one sees a plaque on the wall that reads:

In 1898, in a laboratory of this school, Pierre and Marie Curie, assisted by Gustave Bemont, discovered radium. While Henri Becquerel was publishing his works on what he called uranic rays, Marie Sklodowska Curie was thinking about possible topics for her doctoral studies and was reading Becquerel's chronicles. Her husband, Pierre, was an instructor a t the Ecole Municipale de Physique et de Chimie. It was natural for her to do her work there. The topic of the new penetrating rays seemed the most interesting to her, and this is the topic she chose. The head of the Ecole Municipale, Paul Schutzenberger, graciously assigned her a laboratory, but he asked that she provide her own materials. Marie Curie's Contribution Approaching the subject at the time that Becquerel was abandouingit, we might say that Marie Curie changed the question. Beoquerel, being a physicist, asked, "What are the properties of these rays, and where does the energy come from?" On this last question he was completely baffled. Curie had been studying chemistry, so she asked a chemist's questions: "Is uranium the only element that produces these rays? What other substances, if any, give the radiation. and how much?" Her first need was a means of measuring the intensity of the radiation. The ~ h o t o e r.a ~ h.ilate c was much too crude. Even the rate of discharge of a n electroscope was unreliable, for it varied with time and intensity in a complex way. Enter Pierre Curie. Ten years earlier, he and his brother Jacques had worked on piezoelectricity, the production of electric charges by stretching and compressing certain This phenomenon is at the basis of kinds of crystals (4,5). many devices we use today: quartz watches, phonograph pickups, and strain gauges. It is interesting that Becquerel's grandfather had observed piezoeleclricity. Pierre Curie built a device that allowed one to measure the extremely small electric currents that passed through air in the presence of uranium and its compounds. The idea was to compensate the current flowing across the air under an electric potential by nn opposite displacement of charge produced by hanging known weights on a quartz crystal. A quadrant electrometer served as a null instrument to show whether the opposing currents were in balance. The currents measured were in the range of picoamperes.

Marie Curie's Experiments With this apparatus Marie Curie could test one substance aRer another, measuring the current that each pm12

Journal of Chemical Education

Sklodowska Curie, Comptes Rendus 1898,126,1101: Selected Data

Substance

Current, picoamps

Uranium metal (containing carbon) Black uranium oxide, UzOs

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Green uranium oxide, Us08 Yellow uranates of Na, K, NH4 Yellow UOz(N03)z; Green U(S04)z Artificial chalcolite (uranium-copperphosphate) Natural chalcolite mineral Pitchblende, U308, from various sources Autunite mineral (uranium-calciumphosphate)

52 16, 67, 83

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duced in the space between two horizontal metal disks. She spread the substance on the bottom disk in a layer thick enough (about 1 mm) that adding more did not increase the current, and using a battery, applied a potential between the disks of about 100 V. She tested every substance she could lay her hands on. She borrrowed chemicals from several laboratories and minerals from the museum. (Of course she returned them aRer use. Acknowledgements appear as a footnote to her first manuscript.) Henri Moissan gave her some uranium metal. The tests confirmed that the activity was an atomic property, that is, that an active element was active regardless of its chemical combination. Two elements, uranium and thorium, gave the rays; others did not. The currents were roughly proportional to the fraction of the element in the samnle: thus. oure uranium metal vielded about three times th'e c&renkLhmuranyl sulfate. " The minerals gave a great surprise. Uranium metal gave a current of 24 picoamps (see the table). Two samples of pitchblende, a uranium oxide mineral, gave currents 67 and 83 picoamps, respectively! The mineral chalcolite, a beautiful green phosphate of uranium and copper, gave 52 picoarnps, more than twice the current of uranium metal. Chalcolite that Madame Curie prepared in the laboratory, which had exactly the same chemical composition as the mineral, gave only 9 picoamperes. The discrepancies were too large for Marie to ignore. There was only one explanation: the minerals, pitchblende and chalcolite, must contain an element other than uranium. The amount of this element had to be very small, for chemical analysis did not reveal it. Therefore, the element must be very active. Marie and Pierre Curie's Findings At this point Marie Sklodowska Curie presented her results to the Academy through Gabriel Lippmann, an Academy member, who later won the Nobel Prize for work on color photography. The next treatise was written three months later. Pierre Curie was a co-author. He had dropped his own research and joined Marie in hers. The title, "Sur une substance nouvelle radioactive, contenue dans la pechblende", is worth noting, for here is the first use of the adjective "radioactive". This report and the next (see Appendix)were

statement, perhaps the most moving words in all the history of science: Dissolve in nitric acid, pass hydrogen sulfide *Precipitate:sulfides of Pb, Bi, Cu As, Sb

Solution: U, Th

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Ammonium sulfide

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Solution: As, Sb

Insoluble; *sultidesf Pb, Bi, Cu I

Dissolve in nitric acid, add sulfuric acid *solution: Bi, Cu

Solution: Cu

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Precipitate, PbSO4

*Precipitate: Bi(OH13. convert to sulfide. h e a t i n cacuum: Bi remsins; 'HI.ACK 301.lD CONDENSES IN COOLER PART OF TUBE

Afiowchart of the Curies'work. P. Curie and S. Curie, Comptes Rendm, 1898, 127, 175.

presented to the Academy by Heuri Becquerel, who collaborated fully with the Curies. They were looking for the new, highly radioactive element that was contained i n ~itchbleude.Marie was a chemist, so she started out witg the traditional qualitative analysis scheme. due to Fresenius. which was old even in her time. Until rkcently we used to teach it to all our chemi s t students: ~ the "march of the cations". First the minerai pitchblende was dissolved in nitric acid, then smelly hydrogen sulfide was passed. (See the figure.) Ablack precipitate appeared. When this was treated with ammonium sulfide, some of it dissolved. The part that did not dissolve in ammonium sulfide was dissolved in nitric acid. Sulfuric acid was added to precipitate lead, and ammonia was added to precipitate blsmuth. At each step, the precipitate and the solution were tested for radioactivity with Pierre Curie's electrometer device. Fractions that showed no activity were discarded. The final precipitate of bismuth hydroxide was very radioactive. I t was converted to sulfide, dried, and heated in a vacuum. Bismuth sulfide stayed behind, but a vapor rose from the heated material and condensed a s a black film in the cooler part of the tube. The radioactivity was here. Here was the new element! A New Element

The Curies were reluctant to claim this black solid as a new element. To make this claim they needed more than the radioactivity. Their kind and obliging colleague, Mr. Demar~ay,produced an emission spectrum of the solid and could find no new line that would indicate a new element. So the Curies contented themselves with the following

We therefore think that the Bubstance that we have extracted from pitchhlende contains a metal previously unknown, a neighbour of bismuth in its analytical properties. If the existence of the new metal is confirmed, we propose to call it polonium, after the native land of one of us. The next monograph was presented on 26 December 1898. The authors were Mr. Pierre Curie, Madame P. Curie, and Mr. G. BBmont. BBmont was the manager of the laboratories of the School. The report focused on a n active, new substance that they had somehow missed in their first investigation. The activity was associated with the element barium. Barium chloride recovered from pitchblende was like common barium chloride exceDt for its radioactivity. To concentrate the radioactive fraction, the Curies used fractional crvstallization. testine the ~ r o d u c t swith the electrometer a t every step. ~ i r s c t h got e ~ a product that was 60 times a s active a s uranium metal; then, one that was 900 times a s active. There they had to stop, because they had run out of material. Announcement of the Discovery of Radium Nevertheless, they felt confident in announcing the discovery of another new element, which they called "radium". They reported,

Monsieur Demar~ayagreed t o examme the rpeetmm ofnur substance, with a wlllinpes~for which w e shall ncvcr be able to thank him enough. He found a new line in the spedrum, which was hardly visible in the product that was 60 times as active as uranium, but was quite strong in the chloride whose activity was 900 times that of uranium. The intensity of this line increases alone with the radioactivity. and we think this is a good reason to artribute thisline ta the radioactive fraction of our material. Thev tried to measure the atomic weieht of the new element,"but the value they found was so ;lose to that of barium that t h e could ~ onlv sav. "The new substance still has a very high Goportion i f b&um. . . . The radioactivity of radium must thus he enormous". Their most active fraction emitted just enough light to make the Curies wonder. as had Becouerel. about the origin of the energy. One idea was t h a t t h e dhole universe was crisscrossed by a kind of radiation previouslv unknown, a n d t h e heavy atoms of u r a n k m , thoiium, polonium, and radium had the power to capture this radiation. This phenomenon would violate thk second law of thermodynamics. Or was the first law, the law of conservation of energy, inadequate? To proceed, the Curies needed more raw material. They informed their readers, in a footnote to their treatise of December 26, that a corresponding member of the Ecole, Mr. Suess, a professor a t the University of Vienna, had kindly persuaded the Austrian government to send 100 kg of the residue from pitchblende treatment a t the Joachimsthal mine. This was waste material from which the uranium had been removed. The sequel is described in all the biographies of Madame Curie. The residues did arrive and were worked up, with hard, backbreaking physical labor, in a n old shed that belonged to the Ecole. Today, in the courtyard, ifyou can find it amongthe parked cars, you can see a line of white bricks, set into the pavement, that marks where the shed stood. Extraction of Radium from Pitchblende Details of the extraction of r a l u m from pitchhlende renidues arc given in Madame Curie's doctoral thcsia (fi Therc was so little radium in the residues thnt 100 kc were not enough. They needed tons and received them compli-

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Volume 69 Number 1 January 1992

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ments of the Austrian government. There was too much material to handle in the shed, so a chemical company kindlv did the first staee of the treatment. Financial aid -came from several sources, including an anonymous donor. The Institut de France, that aumst agency that administers the Academy of sciences &d fou; other learned societies in the name of the Republic, contributed 20,000 francs. The orocess was lone and compiicated. At Joachimsthal the pit&blende was roasted with sodium carbonate in contact with air. The fused mass was extracted first with water, then with dilute sulfuric acid. Uranium went into solution. first as its carbonate complex, then as its sulfate complex; The solid that was left was what was sent to the Curies. I t contained silica, iron, and aluminum oxides, lead sulfate, and small amounts of many elements, including barium and radium, bismuth and polonium, and a new radioactive element called actinium, discovered by Debierne, a friend of the Curies at the Sorbome. Barium and radium were present as their highly insoluble sulfates. The first step in treating the residues was to boil with concentrated sodium cnrhonate solut~on.Inis treatment converted barium and radium sulfates to their carhmates. The carbonates are 10 times as soluble as the sulfates (see tables of solubility products), but the large excess of sodium carbonate pushed the wnversion to completion. Sodium sulfate was washed out with water, then the carbon a t e s were dissolved i n dilute hydrochloric acid. (Freshman can be interested in these experiments.) The cvcle. insoluble sulfate to insoluble carbonate to soluble Chlohde and back to insoluble sulfate again by adding sulfuric acid, was repeated until barium and radium had been separated from everyhng else. There were other chemical steps; thus, at the start, lead, bismuth, and accompanying polonium were precipitated by hydrogen sulfide. Actinium was oreci~itatedwith iron. aluminum, and the rare earths by aiding ammonia.

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Separation of Radium and Barium Now the radium and barium had to be separated from one another. This was done hy a long seriesof fractional crvstallirations. The proeress of enrichment was followed b; the electrometer, b k soon the activity became so strong that the piezoelectric crystal could not handle it. So the Curies measured the atomic weight. They did this by wnverting a known weight of metal chloride to silver chloride and weighing the silver chloride-by the traditional, tedious gravimetric method. The measured atomic weight rose as the radium fraction rose, and levelled offa t a value of 206. Finally, they had pure radium chloride. The purity was confirmed by emission spectra run by their friend Demarpy. A ton of residues had given a tenth of a gram of pure radium chloride! (They had lost some along the way. Later estimates gave 170 mg radium per ton of pitchblende.) The 1903 Nobel Prize WinnersBecquerel, Curie and Curie I n 1903 the Nobel Prize in physics was awarded to Becquerel, Curie and Curie for the diswvery of "spontaneous radioactivity". As more radium became available, more experiments in giving became possible. The Curies were very samoles of their oroducts to scientists who askedfor them. ~ecquerel,inspired by the Curies and taking advantage of their intense radiation sources, went back to the laboratory and made more studies of the new rays. He observed their deflection in a magnetic field and saw that there were a t least two kinds (remember, he had been studying the Zeeman effect) of rays. One kind, the kind that could be 14

Journal of Chemical Education

deviated, he identified with "cathode rays", that is, highsoeed electrons. These are what we now call "beta-rays". his phase of Becquerel's work, made in close collaboration in 1900-1901. with the Curies (7).resulted in seven -papers (See Appendix.) Marie Curie in her thesis refers to the work of J. J. Thomson, Rutherford and Soddy in England, and to the recognition of three kinds of rays, alpha-, beta-, and gamma. Alpha-particles are so much more massive than betaparticles (electrons) that they are deviated less, and in the early experiments they did not seem to be deviated a t all. The intensity of the radiation could not he changed by heating radium to a red heat, nor by cooling i t in liquid air. A temporary lowering of the activity occurred when a radium salt was dissolved in water and recrystallized, and also on heatine the material in a n open vessel, but the lost adivity came i a c k in a few days weeks (8).Becquerel had noted the same effect when he dissolved a uranium salt in water and recrystallized it. An explanation of these effects was soon to come from the studies of Frederick Soddy on the radioactive disintegration series, and the recognition of easeous emanations from radium and thorium. A measurement made with a gram of enriched radium chloride (116 radium, 516 barium) by P. Curie and A. Lab o d e in 1903 (9)was the rate of production of heat. It was enormous. One gram-atomic weight of radium salt produced the same amount of heat every hour as did the burning of one gram-atomic weight of hydrogen, and the heat production showed no signs of ever slowing down; it went on and on. Where did all this energy come from? They wrote:

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If one seeks the origin of this heat in an internal transformation, this transformationmust be of a very profound nature and caused by a modification of the radium atoms themselves. The energy released in atomic transformation must be exceedingly large. The Origin - of the Energy . . What was the origin of the energy? Finally came a reply to this ouestion. from Albert Einstein, who was working in a total& different area. In a three-page article (10)entitled, "Does the inertia of a body depend on its energy coutent?" emer ed the famous equation, E = M e ,but it read as L = MV $. "If a body gives out energy L in the formof radiation, its mass is reduced by LN '. "In his nehto-last paragraph Einstein notes, "It is not out of the question that bodies whose energy is very variable, like radium salts, may provide a proof of this theory". The quantity V or C is the speed of light, and it is a large number. Its square is much larger. Thus, it only takes the annihilation of a very little mass, unweighably small, to produce an enormous amount of energy The energyof radioactivity originates here. Yes, the law of conservation of energy did need to be modified. Todav we recoenize the biological effects of high-energy radiatibn, and we go to great l